1. Field of the Invention
The present invention relates to a method of adding crystallization promoting catalyst elements to a semiconductor material partially or entirely consisting of an amorphous component, or a substantially intrinsic polycrystal semiconductor material, subjecting the semiconductor material to an annealing process or the like, to thereby improve the crystallinity of the semiconductor material, and further efficiently moving the crystallization promoting catalyst elements to a region where the semiconductor device is not adversely affected by the crystallization promoting catalyst elements.
2. Description of the Related Art
In recent years, a research on a technique of lowering a temperature during a semiconductor device process has been extensively advanced. The main reason is because the semiconductor device needs to be formed on an insulating substrate made of glass or the like which is inexpensive and rich in processability.
A melting point of glass widely and generally used as the substrate of a semiconductor film is about 600.degree. C., and the temperature of the substrate cannot be made higher than 600.degree. C. Therefore, the semiconductor device must be manufactured at a temperature lower than about 600.degree. C.
In the semiconductor process, there may be required that the amorphous component contained in the semiconductor material or an amorphous semiconductor material is crystallized, or that the crystallinity is improved.
Up to now, thermal annealing has been made to satisfy the above requirements. In the case where silicon is used as the semiconductor material, annealing is conducted at a temperature of 600 to 1100.degree. C. for 0.1 to 48 hours or more hours, to thereby crystallize the amorphous material, to improve the crystallinity and so on. When the thermal annealing is intended to be thus conducted on a semiconductor film formed on a glass substrate, since the crystallinity must be improved at a temperature extremely close to a strain point of the glass substrate, that is, at about 600.degree. C., a very long time is required for crystallization. The temperature of 600.degree. C. is close to the lowest temperature required for the crystallization of silicon, and when the temperature is 500.degree. C. or lower, no crystallization occurs even if annealing is continued for a long period of time.
Therefore, there is required that the conventional crystallizing process is reconsidered from the viewpoint of making the process temperature low. The laser beam irradiation technique is one of techniques that realize a low-temperature process. This is because a laser beam can give a high energy equal to thermal annealing at about 1000.degree. C. restrictively to only a semiconductor film so that the entire substrate does not need to be exposed to a high temperature. The irradiation of a laser beam is mainly made by a method in which a large-energy laser pulse is irradiated onto the semiconductor material using a pulse oscillation laser such as an excimer laser so that the semiconductor material is instantaneously melted and solidified, to thereby crystallize the semiconductor material. The crystallinity of the semiconductor film obtained by crystallization due to the laser of this type is relatively high.
However, the characteristic of a TFT manufactured using a silicon film thus crystallized by the laser is varied by instability of the laser, which requires further review.
Under the above circumstances, the present inventors have devised a method in which nickel which is one of elements that promote the crystallization is added to amorphous Si deposited through the plasma CVD method, and thermal annealing is then conducted on the amorphous Si to make the amorphous Si grow in a solid layer. This method enables the amorphous Si to be changed into a polycrystal Si film under a low-temperature and short-period condition, that is, at 600.degree. C. for 4 hours.
The above method using nickel enables the polycrystal Si film to be formed on the glass substrate without using the laser for a relatively short period of time. However, disappointedly, the characteristic of the polycrystal Si film is not satisfactory in comparison with a film obtained through the laser. Concentrations of nickel are found everywhere in the interior of the film obtained by the above method in which nickel is added to the amorphous Si. When a portion where nickel is concentrated is unexpectedly in a channel region or a high-resistant region (for example, a portion called "offset region") of the TFT, the characteristic is significantly degraded. In particular, an off-state current is remarkably increased.
From the above viewpoints, the present inventors reviewed a method of removing nickel from a film in which nickel is contained, or reviewed a method of removing nickel from at least the channel region and the high-resistant region. Then, the present inventors have paid attention to a phenomenon in which annealing is conducted on a nickel contained film to which phosphorous is selectively added, to thereby allow nickel to be attracted to phosphorus so that nickel is substantially removed from a region other than a region where phosphorus is added, and tried to optimize the condition of the phenomenon. A technique of removably sucking the impurities is generally called "gettering".
The above method is usually that, typically, after phosphorus is added to the film, thermal annealing is conducted at about 600.degree. C. for several hours to ten several hours so that nickel and other impurities are removably attracted to phosphorus. At a temperature of 600.degree. C., this method utilizes a property that nickel is remarkably moved although phosphorus hardly moves in the film.
Although a sufficient effect is obtained even by the above method, a long period of time is required for processing. Also, an area of the region where phosphorus is added is required to increase, thereby leading to a problem that the fining of a circuit is adversely affected by such requirement, or the like. In particular, the gettering technique that imposes restrictions on the fining of the circuit goes against an advance in technique.